1. Field
The present invention relates to an ionization device and a mass spectrometry apparatus for ionizing a specimen and mass analyzing a specimen.
2. Description of the Related Art
For analysis of component in a surface of a solid specimen, a technology for ionizing a solid substance in an atmospheric pressure environment has been developed.
For example, such a technique is described in the following non-patent literature: Yoichi Otsuka et al., “Scanning probe electrospray ionization for ambient mass spectrometry” Rapid Communications in mass spectrometry, 26, 2725 (2012). In the technique, a small volume of the solvent is deposited onto a microregion of a surface of a solid specimen, and a component of the specimen is dissolved in the solvent. Thereafter, the component is ionized by electrospray ionization. Generated ions are introduced into a mass spectrometry apparatus, which measures the mass-to-charge ratio of the ion. Thus, the component can be analyzed. To deposit the solvent onto the microregion of the surface of the solid specimen, a probe formed from a needle-like capillary is used. The solvent is continuously fed to the probe. A liquid bridge is formed between the probe and the surface of the solid specimen that is located in close proximity to the probe. The component contained in the surface of the solid specimen is dissolved into the liquid bridge. The solvent having the component dissolved therein is ionized by applying a voltage to the solvent. The probe is vibrated and, thus, the solvent that is continuously supplied to the surface of the solid specimen is ionized. Such a technique is referred to as Tapping-mode Scanning Probe Electrospray Ionization (Tapping-mode SPESI). In contrast, a technique for ionizing the solvent with the probe remaining in close proximity to the surface of the solid specimen is referred to as Contact-mode Scanning Probe Electrospray Ionization (Contact-mode SPESI).
In the Tapping-mode SPESI described in non-patent literature above, a liquid bridge is alternately formed and disrupted. In the technique, dissolution of a component into the liquid bridge and ionization of the component are alternately and continuously performed. The frequency of the formation of the liquid bridge and the ionization is determined by the frequency of vibration of the probe. In addition, the mass spectrometry apparatus is electrically separated from an ionization device. The mass spectrometry apparatus and the ionization device are independently driven. The ions introduced into the mass spectrometry apparatus are measured within a predetermined measurement period of time.
In the technique, measurement using mass spectrometry is performed even during a period of time during which ionization is not performed, that is, during a period of time during which a liquid bridge is being formed and during a period of time during which ions are being generated after the liquid bridge is formed.
As a result, a noise signal generated when ionization is not performed is mixed with measurement data, which makes mass spectral analysis of the data difficult.
In addition, it is difficult to finely control the number of ionization processes performed within the measurement time of ionization, the quantitative capability of measurement is low and, thus, the quantitative capability when the measurement values each measured in one measurement process are compared with one another is low.
In contrast, in terms of Contact-mode SPESI, a technique for steadily vibrating a substrate having a solid specimen placed thereon is proposed. In such a technique, vibration of the substrate makes ionization stable. However, measurement of ions is performed during an entire period of vibration time of the substrate, a noise signal generated during a period of time during which ionization is not actually performed is mixed in measurement data. In addition, when the substrate is steadily vibrated for a long time, a device for generating the vibration is heated. The heat may cause the amplitude and the frequency of the vibration to fluctuate.